Raster imaging systems are common and their use is well documented for many purposes including television and other cathode ray tube screen use and microfilm and microfiche imaging. It is typical for known raster imaging systems to employ full frame exposure and therefore full frame focusing optics. This requires that the entire frame, which may be as large as 4" by 6", be within tolerance at the focal plane of the scanning system, which is difficult. Then there must be some means for transporting the media to the next full frame exposure. This can waste time in accelerating the media, transporting the media without imaging, decelerating the media and waiting for it to settle before commencing the next frame exposure.
The precision of motion and focus is particularly important to the computer output microfilm (COM) industry. In making microfiche and related types of images it is conventional to hold the media, typically a film, while scanning in two directions. That is what is normally understood by the term "raster scanning."
Linescan imaging systems use an imaging beam or beams which are modulated with the desired data and scanned across a media while the media is moved with a constant velocity perpendicular to the beam sweep. This provides a two dimensional image. Presently, constant velocity transports for moving discrete media, such as sheets of film or paper, silicon wafers, printer's plates, among others, must not only provide the constant velocity, but also must hold the entire media in the focal plane to be ready for imaging.
Other systems which employ a continuous media require elaborate apparatus to hold the media precisely at the focal plane while the media is moving. One example of such a structure is an air or gas bearing to position film away from the aperture as described in U.S. Pat. No. 4,168,506. This technique not only has the drawback of requiring precision orifices, well regulated and filtered gas, and other precision aspects, but is not applicable to discrete media.
When it is necessary to start and stop the recording media between full frame exposures, as in the prior art, there is overhead time or lost time in the exposure-to-exposure moves during which no exposure can be accomplished. Further, by requiring scan motion in two directions and media motion in order to expose subsequent frames, there is substantial complexity and cost involved in such an imaging system. Because the aperture is a full frame opening in the prior art system, it is difficult to prevent fogging from stray scattered energy. Also in the previous systems it was generally impossible to have negative imaging due to image blooming from stray scattered energy because of the size of the full frame opening caused by the same problem related to fogging as mentioned above. Because the two dimensional motion was previously required for full frame imaging, it was not possible to use a relatively simple linear motion to accomplish the scanning necessary.